Color-stable, ITO-free white organic light-emitting diodes with enhanced efficiency using solution-processed transparent electrodes and optical outcoupling layers

In this work, we demonstrate color-stable, ITO-free white organic light-emitting diodes (WOLEDs) with enhanced efficiencies by combining the high-conductivity conducting polymer PEDOT:PSS as transparent electrode and a nanoparticle-based scattering layer (NPSL) as the effective optical out-coupling layer. In addition to efficiency enhancement, the NPSL is also beneficial to the stabilization of electroluminescent spectra/colors over viewing angles. Both the PEDOT:PSS and the NPSL can be fabricated by simple, low-temperature solution processing. The integration of both solution-processable transparent electrodes and light extraction structures into OLEDs is particularly attractive for applications since they simultaneously provide manufacturing, cost and efficiency advantages. PostprintPeer reviewe

Coupled plasmonic modes in organic planar microcavities

We report on the nature of the resonant modes in organic planar microcavities featuring semi-transparent metallic layers. We theoretically demonstrate that symmetric microcavities support a total of four modes originating from the coupling of surface plasmon polaritons. For red top-emitting organic light-emitting diodes with one semi-transparent metallic electrode, we identify three coupled plasmonic modes and calculate a light outcoupling efficiency close to 34% when assuming emitters with isotropic transition dipole moment. This value is estimated to increase to 50% in the case the dipole moment is purely horizontal. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729820]</p

p-Channel Field-Effect Transistors Based on C-60 Doped with Molybdenum Trioxide

Fullerene (C-60) is a well-known n-channel organic semiconductor. We demonstrate that p-channel C-60 field-effect transistors are possible by doping with molybdenum trioxide (MoO3). The device performance of the p-channel C-60 field-effect transistors, such as mobility, threshold voltage, and on/off ratio is varied in a controlled manner by changing doping concentration. This work demonstrates the utility of charge transfer doping to obtain both n- and p-channel field-effect transistors with a single organic semiconductor.close9

Singlet exciton diffusion length in organic light-emitting diodes

We present a simple and accurate method to determine the singlet diffusion length in an operating organic light-emitting device (OLED). By using electrical rather than optical excitation, the method ensures that excitons are formed in a tightly confined generation zone, from which they can diffuse towards a quenching material. For a series of devices with varying distance between generation and quenching region, different emission intensities are found, and the experimentally obtained emission spectra of these devices can be used to determine the singlet diffusion length in the emissive layer of the device. By carefully choosing OLED layer materials and thicknesses, we can ensure well-defined quenching and blocking boundary conditions and exclude cavity effects as well as emission from the quenching material. An analytical model is developed to analyze the emission intensity found experimentally. We show that disregarding the fact that the generation zone has a nonzero width leads to an overestimation of the diffusion length. Furthermore, the current, i.e., the excitation density dependency of the singlet diffusion length, is investigated. At low current density (0.15 mA/cm(2)), a singlet diffusion length of 4.6 +/- 0.5 nm is obtained in N,N'-di-1-naphthalenyl-N,N'-diphenyl-[1,1':4',1 '':4 '',1'''-quaterphenyl]-4,4'''-diamine (4P-NPD). The singlet diffusion length decreases to 4.0 +/- 0.5 nm at 154.08 mA/cm(2).</p

A high performance liquid chromatography method to determine phenanthroline derivatives used in OLEDs and OSCs

A high performance liquid chromatography (HPLC) method to detect commonly used electron transport layer (ETL) and hole blocking layer (HBL) materials in organic light emitting diodes (OLED) and organic solar cells (OSC) was established and optimized. The novel method enables the investigation of degradation processes of a range of ETL and HBL materials that were previously difficult to discriminate using the HPLC technique. For instance, 4,7-diphenyl-1,10-phenanthroline (Bphen) and 2,9-dimethyl-4.7-diphenyl-1,10-phenanthroline (BCP), which are commonly used in OLEDs and OSCs. as well as the new phenanthroline derivatives 2,4,7,9-tetraphenyl-1,10-phenanthroline (TPphen), (2-naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (HNBphen) and 2,9-di(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (NBphen) were distinguished. The detection of these molecules was achieved on a phenyl column using an acetonitrile/water mixture (78:22, v/v%) as mobile phase and a photodiode array detector (PDA) for absorbance detection. The factors affecting peak tailing were studied and optimized. The addition of triethylamine (TEA) to the mobile phase after flushing the HPLC system with ethylenediaminetetraacetic acid disodium salt (Na(2)EDTA) showed the best reduction of the tailing effect. Furthermore, the influence of the tailing on the calculation of the peak area from HPLC chromatograms was investigated. (C) 2012 Elsevier B.V. All rights reserved.</p

Analysis of the external and internal quantum efficiency of multi-emitter, white organic light emitting diodes

We report on a theoretical framework for the efficiency analysis of complex, multi-emitter organic light emitting diodes (OLEDs). The calculation approach makes use of electromagnetic modeling to quantify the overall OLED photon outcoupling efficiency and a phenomenological description for electrical and excitonic processes. From the comparison of optical modeling results and measurements of the total external quantum efficiency, we obtain reliable estimates of internal quantum yield. As application of the model, we analyze high-efficiency stacked white OLEDs and comment on the various efficiency loss channels present in the devices. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4757610]</p